David Hobill

Short-Period Binary Stars: Observations, Analyses, and Results

Compact Relativistic Binary Systems.- Black Hole Binaries: The Journey from Astrophysics to Physics.- Searches for Gravitational Waves from Binary Neutron Stars: A Review.- Observations of the Double Pulsar PSR J0737-3039A/B.- Gravitational Lensing in Compact Binary Systems.- Accreting Neutron Star Binaries.- Accreting Neutron Stars in Low-Mass X-Ray Binary Systems.- Observations and Modeling of Accretion Flows in X-ray Binaries.- Cataclysmic Variable Systems.- Modeling the Hot Components in Cataclysmic Variables: Info on the White Dwarf and Hot Disk from GALEX, FUSE, HST and SDSS.- The Cool Components in Cataclysmic Variables: Recent Advances and New Puzzles.- Models for Dynamically Stable Cataclysmic Variable Systems.- Modeling Short-Period Eclipsing Binaries.- Distance Estimation for Eclipsing X-ray Pulsars.- The Tools of the Trade and the Products they Produce: Modeling of Eclipsing Binary Observables.- The Closest of the Close: Observational and Modeling Progress.- Aspects of Short-Period Binary Evolution.- Common Envelope Evolution Redux.

Head-on collision of two equal mass black holes

(February 7, 2008)We study the head-on collision of two equal mass, nonrotating black holes. Various initialconfigurations are investigated, including holes which are initially surrounded by a common apparenthorizon to holes that are separated by about 20M, where M is the mass of a single black hole. Wehave extracted both l = 2 and l = 4 gravitational waveforms resulting from the collision. Thenormal modes of the final black hole dominate the spectrum in all cases studied. The total energyradiated is computed using several independent methods, and is typically less than 0.002M. We alsodiscuss an analytic approach to estimate the total gravitational radiation emitted in the collision bygeneralizing point particle dynamics to account for the finite size and internal dynamics of the twoblack holes. The effects of the tidal deformations of the horizons are analysed using the membraneparadigm of black holes. We find excellent agreement between the numerical results and the analyticestimates.PACS numbers: 04.30.+x, 95.30.Sf, 04.25.DmI. INTRODUCTION

Possible physical realizations of the Tolman VII solution

The Tolman VII solution (an exact static spherically symmetric perfect fluid solution) to the Einstein equations is reexamined, and a closed form equation of state (EOS) is deduced for the first time. This EOS allows further analysis leading to a viable model for compact stars to be obtained. Explicit application of causality conditions places further constraints on the model, and recent measurements of masses and radii of neutron stars prove to be within the predictions of the model. It is found that the adiabatic index

Image formation in weak gravitational lensing by tidal charged black holes

{\gamma} \geq 2

A BRIEF REVIEW OF "DETERMINISTIC CHAOS IN GENERAL RELATIVITY"

, and self-bound crust solutions are not excluded. The solution is also shown to obey known stability criteria. It is argued that this solution provides realistic limits on models of compact stars.

Weak gravitational lensing by compact objects in fourth order gravity

We derive a generic weak lensing equation and apply it for the study of images produced by tidal charged brane black holes. We discuss the similarities and point out the differences with respect to the Schwarzschild black hole weak lensing, to both first- and second-order accuracy, when either the mass or the tidal charge dominates. In the case of mass-dominated weak lensing, we analyze the position of the images, the magnification factors and the flux ratio, as compared to the Schwarzschild lensing. The most striking modification appears in the flux ratio. When the tidal charge represents the dominating lensing effect, the number and orientation of the images with respect to the optical axis resembles the lensing properties of a Schwarzschild geometry, where the sign associated with the mass is opposite to that for the tidal charge. Finally it is found that the ratio of the brightness of the images as a function of image separation in the case of tidal charged black holes obeys a power-law relation significantly different from that of Schwarzschild black holes. This might provide a means for determining the underlying spacetime structure.

The Dynamics of Root Growth: A Geometric Model

A short review of chaotic behavior and a dynamical systems approach to time-dependent solutions to the Einstein equations is provided in order to give an overview of the problems that have been studied and the approaches used to understand them.

Gravitational Lensing in Compact Binary Systems

We discuss weak lensing characteristics in the gravitational field of a compact object in the low-energy approximation of fourth order f(R) gravity theory. The particular solution is characterized by a gravitational strength parameter

Numerical simulations of strong gravitational fields

\sigma

Continuous Time Dynamics and Iterative Maps of Ellis-MacCallum-Wainwright Variables

and a distance scale